1. export it to cmp and mat
2. add a new non_targetable object to the solararch.ini (just copy/paste one of the existing ones)
3. now edit the 2 lines to the cmp and mat
4. open the cmp in utf and add hardpoints or remove the empty hardpoint node
5. resize the asteroid to the size that you want with the FLmodelcloner
6. add your new asteroid model to the game with FLE (it should be in the list now)